1. Field of the Invention
This invention relates to a fuel cell power system comprising a fuel cell stack, fuel processor and heat exchangers. More particularly, this invention relates to a fuel cell power system in which the fuel cell stack, fuel processor and heat exchange components are disposed in one thermally integrated assembly. The fuel cell power system of this invention is particularly suitable for use with solid oxide fuel cells and solid oxide fuel cell stacks.
2. Description of Related Art
A fuel cell is an electrochemical device in which the chemical energy of a reaction between a fuel and an oxidant is converted directly into electricity. The basic fuel cell unit comprises an electrolyte layer in contact with a porous anode and cathode on either side. In a typical fuel cell, a gaseous or liquid fuel is continuously fed to the anode electrode, sometimes referred to as the fuel electrode, and an oxidant, such as oxygen from air, is continuously fed to the cathode electrode, sometimes referred to as the air electrode, and electrochemical reactions occur at the electrodes to produce an electric current. Due to the limited electricity generating capacity of individual fuel cell units, a plurality of fuel cell units are typically stacked one on top of another with a bipolar separator plate separating the fuel cell units between the anode electrode of one fuel cell unit and the cathode electrode of an adjacent fuel cell unit.
There are a number of different fuel cell types which are classified based upon a variety of categories including the combination of type of fuel and oxidant, whether the fuel is processed external to or inside the fuel cell, the type of electrolyte, e.g. solid oxides, phosphoric acid, molten carbonate and proton exchange membranes, the temperature of operation and whether the reactants are provided to the fuel cell by internal or external manifolds. The system of this invention is particularly suitable for use in connection with solid oxide fuel cells, which, of the common known fuel cell types, have the highest operating temperatures, in the range of about 800° C. to about 1000° C. The benefits of using higher operating temperature fuel cells include the possibility of using a greater variety of fuels, including CO and methane.
However, a significant issue in the operation of high temperature fuel cells is heat management, in particular minimizing the amount of heat loss. Conventional fuel cell power systems for operation of high temperature fuel cell stacks are limited in thermal integration for heat recovery because of the use of discrete heat exchangers, which require extensive ducting and thermal insulation. This approach has made these fuel cell systems both complex and costly to manufacture and tends to place constraints on fuel cell stack design configurations to support the required plumbing system. To address this issue, U.S. Pat. No. 5,612,149 to Hartvigsen et al. teaches a fuel cell module with a fuel cell column having at least one fuel cell stack, mated with the planar wall of a heat exchanger, wherein the fuel cell column and heat exchanger are mounted to a support structure, and which define an air plenum between the fuel cell column and the planar wall of the heat exchanger, thereby eliminating the ductwork and insulation requirements associated with heat exchange systems while increasing the efficiency of the heat exchanger. However, the disclosed design only provides for single stage heating of the oxidant inlet by a single heat exchange which would not raise the ambient air for the oxidant to the required operating temperature range of the solid oxide fuel cell stack due to the very limited surface and residence time to which the gas would be subjected. In addition, other key requirements such as fuel feedstock preheating prior to reformation, heating needs during system start-up from ambient conditions and partial load operations are also not addressed by this disclosure.
U.S. Pat. No. 4,943,494 to Riley teaches porous refractory ceramic blocks arranged in a stack configuration providing both support and coupling means for a plurality of solid oxide fuel cells. The ceramic blocks and the outer steel shell of the structure provide connections for the air, fuel and process effluent flows. One of the main objects of the disclosed structure is to provide a support structure that integrates fuel, air and effluent flow channels for reduction of interconnection complexities for cost reduction and commercial feasibility. However, the disclosed structure does not provide any means for heat recovery, which is critical for efficient operation and cost effective system operation.
U.S. Pat. No. 5,763,114 to Khandkar et al. teaches a thermally integrated reformer located inside of a furnace structure housing solid oxide fuel cell stacks. In this system, heat from the fuel cell oxidation reaction is recovered to support the endothermic reformation reaction. Heat is recovered by heat transfer to the reformer by radiation from the fuel cell stack and by forced convection from the exhausting airflow exiting the furnace. Although addressing the need for heat recovery and transfer to the fuel feedstock as well as support for the reformation reaction, the heating of the air for the oxidant feedstock is not integrated and is provided by external means resulting in system inefficiency and fabrication complexity. An apparatus for heat recovery is also taught by U.S. Pat. No. 5,906,898 to Pondo, which teaches a fuel cell stack with oxidant flow paths between separator plates and along the outside surface of the fuel cell stack for control of the heat generated by the fuel cells. This patent also teaches direct heating of the oxidant feed gas by using recovered heat from the fuel cell stack by way of heat exchange panels mounted externally on the fuel cell stack, providing oxidant inlet flow paths to the fuel cell stack. However, the highest temperature effluent stream is not fully utilized in this configuration for heat recovery because of its containment inside of the fuel cell stack in the oxidant outlet internal manifold.